The present invention generally relates to a bipolar-complementary metal oxide semiconductor circuit, and more particularly to an improvement of a circuit structure of a bipolar-complementary metal oxide semiconductor circuit.
Recently, there has been considerable activity in the development of a bipolar-complementary metal oxide semiconductor circuit (hereinafter simply referred to as a Bi-CMOS circuit). It is well known that CMOS circuits have advantages of high integration density and low power consumption. On the other hand, bipolar transistor circuits have advantages of high driving capability and high mutual conductance. Bi-CMOS circuits have the advantages of both the CMOS circuit and bipolar transistor circuit. A Bi-CMOS circuit includes a logic circuit portion composed of CMOS transistors and a load driving circuit portion composed of two bipolar transistors. A first bipolar transistor of the load driving portion is used for charging up a capacitive load connected thereto. A second bipolar transistor of the load driving portion is used for discharging the capacitive load.
Currently, two types of Bi-CMOS circuits are known. In a first type of Bi-CMOS circuit, when the first bipolar transistor is turned OFF, a base thereof becomes connected to a negative power source through a current path for supplying the second bipolar transistor with a base current produced by a charge stored in the capacitive load connected to the Bi-CMOS circuit. In the second type of Bi-CMOS circuit, when the first bipolar transistor is turned OFF, the base thereof becomes connected to the negative power source through an n-channel MOS transistor which is a part of a complementary MOS inverter in the logic circuit.
However, the conventional Bi-CMOS circuit of the first type has a disadvantage described below. When the first bipolar transistor is turned OFF, a reverse-biased voltage is applied between the emitter and base thereof. This is because the base voltage of the first bipolar transistor quickly decreases to a negative power source voltage, compared with a decrease of the emitter voltage of the first bipolar transistor. When the reverse-biased voltage exceeds a base-emitter breakdown voltage of the first bipolar transistor, it may be damaged.